Precious metal oxidation catalysts are generally used as a means for removing pollutants from the exhaust gas of an internal combustion engine. These oxidation catalysts remove CO, HC, and NO.sub.x simultaneously from engine exhaust gases when the air-fuel ratio of the exhaust gas is at a stoichiometric air-fuel ratio. The ability of oxidation catalysts to remove NO.sub.x in the exhaust gas falls rapidly, however, when the air-fuel ratio of the exhaust gas becomes leaner. Therefore, in engines operating in a lean air-fuel ratio environment, such as a direct injection stratified charge engine (DISC) or other lean-burn engine technologies, it is difficult to remove NO.sub.x from the exhaust gas using a conventional oxidation catalyst.
Accordingly, to achieve NO.sub.x control, exhaust after-treatment systems have included a lean NO.sub.x trap (LNT). Presently, however, the LNT is only a trapping device during the lean operation. The LNT must be purged periodically in order to maintain its level of efficiency. To meet the purge requirement, the DISC or lean burn engine has to run at an air-to-fuel ratio rich of stoichiometry periodically during lean operation. In addition, the LNT has a very narrow operating temperature range beyond which the LNT trapping capacity and efficiency is greatly reduced.
For a DISC engine running in a stratified mode, the LNT is typically purged by running the engine rich of stoichiometry for 2-3 seconds every 50 seconds. The engine control system has to manage this purge cycle without causing noticeable torque disturbance to the vehicle. Prior art approaches to this problem involve the use of an electronic throttle, or other electrical supplemental torque devices, for example, a combined alternator-starter.